1. Field of the Invention
The present application relates to medical devices and, more particularly, to methods and apparatus for spinal stabilization.
2. Description of the Related Art
The human spine is a flexible weight bearing column formed from a plurality of bones called vertebrae. There are thirty three vertebrae, which can be grouped into five regions (cervical, thoracic, lumbar, sacral, and coccygeal). Moving down the spine, there are generally seven cervical vertebra, twelve thoracic vertebra, five lumbar vertebra, five sacral vertebra, and four coccygeal vertebra. The vertebra of the cervical, thoracic, and lumbar regions of the spine are typically separate throughout the life of an individual. In contrast, the vertebra of the sacral and coccygeal regions in an adult are fused to form two bones, the five sacral vertebra which form the sacrum and the four coccygeal vertebra which form the coccyx.
In general, each vertebra contains an anterior, solid segment or body and a posterior segment or arch. The arch is generally formed of two pedicles and two laminae, supporting seven processes—four articular, two transverse, and one spinous. There are exceptions to these general characteristics of a vertebra. For example, the first cervical vertebra (atlas vertebra) has neither a body nor spinous process. In addition, the second cervical vertebra (axis vertebra) has an odontoid process, which is a strong, prominent process, shaped like a tooth, rising perpendicularly from the upper surface of the body of the axis vertebra. Further details regarding the construction of the spine may be found in such common references as Gray's Anatomy, Crown Publishers, Inc., 1977, pp. 33-54, which is herein incorporated by reference.
The human vertebrae and associated connective elements are subjected to a variety of diseases and conditions which cause pain and disability. Among these diseases and conditions are spondylosis, spondylolisthesis, vertebral instability, spinal stenosis and degenerated, herniated, or degenerated and herniated intervertebral discs. Additionally, the vertebrae and associated connective elements are subject to injuries, including fractures and torn ligaments and surgical manipulations, including laminectomies.
The pain and disability related to the diseases and conditions often result from the displacement of all or part of a vertebra from the remainder of the vertebral column. Over the past two decades, a variety of methods have been developed to restore the displaced vertebra to their normal position and to fix them within the vertebral column. Spinal fusion is one such method. In spinal fusion, one or more of the vertebra of the spine are united together (“fused”) so that motion no longer occurs between them. The vertebra may be united with various types of fixation systems. These fixation systems may include a variety of longitudinal elements such as rods or plates that span two or more vertebrae and are affixed to the vertebrae by various fixation elements such as wires, staples, and screws (often inserted through the pedicles of the vertebrae). These systems may be affixed to either the posterior or the anterior side of the spine. In other applications, one or more bone screws may be inserted through adjacent vertebrae to provide stabilization.
U.S. Patent Publication 2004/0127906 (U.S. patent application Ser. No. 10/623,193, filed Jul. 18, 2003) entitled “METHOD AND APPARATUS FOR SPINAL FUSION” describes a bone fixation screw and technique used to secure two adjacent vertebra to each other in trans-laminar, trans-facet or facet-pedicle (e.g., the Boucher technique) applications. This publication is incorporated herein by reference in its entirety. For example, in a trans-facet application, the fixation device extends through a facet of a first vertebra and into the facet of a second, typically inferior, vertebra. In a trans-laminar application, screws, the fixation device, extend through the spinous process and facet of a first vertebra and into the facet of a second, typically inferior, vertebra. In a facet-pedicle application (e.g., the Boucher technique), the fixation device extends through the facet of a first vertebra and into the pedicle a second, typically inferior, vertebra. These procedures are typically (but not necessarily) preformed with bilateral symmetry.
Notwithstanding the success of the above described devices and methods, there are certain challenges associated with applying the trans-laminar, trans-facet or facet-pedicle (e.g., the Boucher technique) techniques to the cervical portion of the vertebrae. For example, due to the anatomy of the cervical region and interference due to the back of the head in a trans-facet approach, the fixation device may need to extend along an axis that, when extended, interferes with the back of the patient's head. For example, FIG. 1 illustrates a portion of the cervical region and a cannulated access device, which extends over the desired entry axis of the fixation device (not shown). As shown, the back of the patient's spine can interfere with the insertion of the fixation device and the various tools needed to insert the fixation device.